Device and method for noninvasive continuous determination of physiologic characteristics

ABSTRACT

The invention comprises devices and methods for the noninvasive monitoring of a physiologic characteristic of a patient&#39;s blood, such as blood pressure. One embodiment is a tissue probe combined with a position sensor for determining the relative height of the probe compared to a level corresponding to the patient&#39;s heart. Alternatively, the tissue probe can be combined with a movement generator for inducing a height change of the probe with respect to patient&#39;s heart. By measuring absorbance characteristics of the blood at varying positions relatively to the level of the patient&#39;s heart, characteristics such as arterial and central venous blood pressure and cardiac output can be determined. In yet another embodiment, two probes are used to compute pulse delays between coupled tissues or opposing tissues. Measuring delays in pulse arrival times in coupled organs or members on opposite sides of the body allows the determination of various physiological characteristics.

This application claims benefit of provisional application Ser. No.60/158,097 filed on Oct. 7, 1999.

FIELD OF THE INVENTION

The present invention relates generally to noninvasive methods ofquantitatively determining various physiologic parameters relating tocardiovascular and respiratory function. More particularly, theinvention relates to a method and apparatus for continuous, noninvasivedetermination of: arterial blood pressure, venous pressure, arterialoxygen saturation, venous oxygen saturation, arterial pulse wavevelocity, aortic pulse wave velocity, aortic pulse flow velocity,cardiac stroke volume, cardiac output, heart rate, and respiratory rate.

BACKGROUND OF THE INVENTION

Critically ill and seriously injured patients require constant care andattention. Doctors, nurses, and hospital technicians need a continuousflow of information about the many patients under their care. Heart rateand blood pressure measurements are two primary vital signs thatindicate the health of patients under their care. When these two commonindices of wellness fall below normal readings, a patient is usually indistress and requires immediate attention.

Dangerous conditions brought about by a cardio-vascular or pulmonarydisease, severe trauma, or drug abuse may bring about a failure of thelungs and heart to supply the bloodstream with life-giving oxygen. Sucha fatal deficiency can be detected by continually gauging the amount ofhemoglobin in the bloodstream that is carrying oxygen. This third vitalsign, which manifests oxygen saturation of the blood, is especiallycritical because a rapid decline in oxygen in the bloodstream isassociated with increased risk of patient mortality.

It is well known that blood pressure can be directly measured by placinga fluidfilled catheter directly into the vessel and coupling this to anelectromechanical transducer. This is the most accurate means, but hasall the disadvantages of invasive measurement, including pain oninsertion, risk of infection or disease transmission, risk of bleedingor thrombosis, and great expense. A further disadvantage is the creationof toxic medical waste (needle, gloves, skin dressing, etc).

Blood pressure measurement can also be measured indirectly using anocclusive cuff (with either auscultation or oscillometry to make thedetermination). This is the most common means of blood pressuremeasurement. Illustrative are U.S. Pat. Nos. 5,582,179, 5,048,533,5,152,296 and 4,793,360.

A further occlusive cuff apparatus is disclosed in U.S. Pat. No.5,766,130. According to the invention, the apparatus includes multiple“pressurized pneumatic cuffs” that are used to “plot blood pressureand/or volumetric blood flow wave forms from a plurality of separatedigits and/or extremities of a patient so that circulatory parametersmay be measured rapidly and recorded from a great number of thepatient's digits or limbs”.

Although commonly employed, the occlusive cuff also has numerousdisadvantages, which include discomfort, intermittent readings, and poorreliability An additional means of determining blood pressure is throughan assessment of “pulse wave velocity”. Several prior art referencesdisclose methods and/or apparatus employing such means. Illustrative isU.S. Pat. No. 5,649,543.

There are also several prior art references that disclose methods and/orapparatus for determining blood pressure through a “pulse waveamplitude” assessment Illustrative are U.S. Pat. Nos. 4,735,213,4,872,461, 4,793,360, and 5,385,149.

Although most of the noted noninvasive blood pressure methods andapparatus, particularly the occlusive cuff, have been employed for manyyears by health care personnel, the conventional methods and apparatushave one major, common drawback-the need for separate calibration.

Accordingly, there is a need for noninvasive methods and devices fordetermining various physiological characteristics, such as centralvenous pressure and cardiac output, without separate calibration. Thereis also a similar need for noninvasive methods and devices fordetermining various blood parameters including pulse amplitude, pulsedelay, pulse velocity, pulse contour, flow velocity and flow delay.

As will be appreciated by one having ordinary skill in the art, thepresent invention satisfies these and other needs.

SUMMARY OF THE INVENTION

The present invention includes a device for the noninvasive monitoringof a physiologic characteristic of a patient's blood. In one embodiment,the device comprises a tissue probe having a radiation emitter and aradiation detector configured to receive the radiation after absorbancethrough the patient's blood; a position sensor for determining therelative height of the probe compared to a level corresponding to thepatient's heart; and a controller for computing the physiologiccharacteristic of the patient's blood based on the absorbance of thefirst wavelength of radiation and the relative height of the probe. Theradiation emitters of the invention can utilize a single wavelength or aplurality of discrete wavelengths and may include visible light,infrared light, and ultraviolet light. The probes are adapted for usewith hands, fingers, feet, toes, ears, earlobes, nares, lips, tongue andthe like. Additional radiation emitters and detectors may also be used.Preferably, the probe further comprises ECG leads.

An alternative embodiment of the device of the invention comprises atissue probe and controller in conjunction with a movement generator forinducing a position change of the probe with respect to a levelcorresponding to the patient's heart. Preferably, the movement generatorinduces a known position change of the probe and moves the probe topositions above and below a level corresponding to the patient's heart.

The invention also comprises method for determining a physiologicalcharacteristic of a patient's blood noninvasively. In one embodiment,absorbance characteristics of the blood are measured at varyingpositions relatively to the level of the patient's heart. By comparingblood parameters such as pulse amplitude, pulse velocity, pulse delay,pulse contour, flow velocity and flow delay to hydrostatic pressuredifferences induced by the position changes, characteristics such asarterial and central venous blood pressure and cardiac output can bedetermined. Alternatively, two probes are used to compute pulse delaysbetween coupled tissues or opposing tissues.

The subject invention relates novel methods for noninvasivedetermination of physiologic characteristics. The first new and uniquemethod and device utilizes changes in hydrostatic pressure induced bypositional changes to facilitate measurements. A second new and uniquemethod and device for noninvasive determination of cardiac output bymeasuring delays in pulse arrival times in coupled organs or members onopposite sides of the body is also described. The two methods are suchthat they can advantageously be used together.

By varying the hydrostatic pressure in an extremity, one can not onlyperform self-calibration for a blood pressure determination, but alsochange the pulse wave velocity and pulse propagation delay with respectto the opposite extremity. With this information pulse wave velocity,and consequently flow wave velocity at the aortic root can bedetermined.

Similar techniques of varying hydrostatic pressure can be used to assessvenous pressure and saturation. The technique of repetitiousdeterminations made while altering position or other variables allows amultitude of additional analyses to be made. The determinations can bemade intermittently or continuously.

Further objects of the invention are exemplified by the followingpotential applications:

(a1). A patient is anesthetized for a surgical procedure. Probes areattached to the index fingers of each hand, and a movement generator isplaced on one arm. A complete set of vital signs and physiologiccharacteristics is generated continuously, including: arterial bloodpressure, venous pressure, arterial oxygen saturation, venous oxygensaturation, arterial pulse wave velocity, aortic pulse wave velocity,aortic pulse flow velocity, cardiac stroke volume, cardiac output, heartrate, and respiratory rate. Other characteristics can be calculated ifdesired.

(a2). A patient is anesthetized for a cardiac surgical procedure. Asaccess to the arms is difficult, probes are attached to the patient'stemples. A complete set of vital signs and physiologic characteristicsis continuously generated.

(a3). A patient is anesthetized for a cardiac surgical procedure; thistime the procedure includes valvular repair or replacement. Since thecardiac output and other characteristics can be continuously computed,the adequacy of the surgical repair can be judged immediately.

(a4). As the number of endoscopic or minimally invasive cardiac surgicalprocedures is expected to increase, the demand for less invasivemonitoring will also increase. The device described herein providesnoninvasive, continuous monitoring of essentially all cardiovascularcharacteristics.

(a5). Cardiac catheterization procedures are often done on criticallyill patients. As the procedures are usually relatively brief andaccomplished without general anesthesia, invasive monitoring methods areoften not desired despite the illness of the patients. The devicedescribed herein will provide the necessary monitoring that is typicallyprovided by much more invasive, expensive, and time consuming monitors

(a6). A patient is hospitalized in the intensive care unit of a hospitalafter a heart attack. Probes are attached to the index fingers of eachhand, and a movement generator is placed on an arm or a leg. A completeset of vital signs and physiologic characteristics can be continuouslygenerated. In addition, arrhythmias can be detected and diagnosed.

(a7). The patient noted above is now moved to a “step-down” or telemetryunit from the intensive care unit. Because the device described hereineliminates the need for invasive monitoring lines, a complete set ofvital signs and physiologic characteristics can still be continuouslygenerated. As the patient has mobility of arms and legs, a movementgenerator is no longer needed, as the patient's spontaneous motion, evenduring sleep, will generate hydrostatic pressures in the limbs, allowingall computations to be made. In addition, the probes may be madewireless, and connected to a central nursing station by means ofinfrared or radio frequency communication.

(a8). The patient noted in applications 6 and 7 above is now moved to aregular hospital bed, and does not require continuous monitoring.However, vital signs can still be recorded by a technician moving thedevice from bedside to bedside on a cart. The device does not requirehighly trained nursing personnel to operate.

(a9). The patient noted in applications 6, 7, and 8 above has now beendischarged from the hospital, and now presents to his physician's officefor follow-up. The same device can be used in physician's offices, as itprovides better care at lower cost. (a10). Ambulances, emergencyvehicles, and military vehicles can also employ this device as it isvery simple to operate, and provides data that currently is impossiblefor them to obtain. In addition, the information can be transmitted tocentral stations where medical personnel are available for help andadvice.

(a11). The device and methods of the invention will provide means ofmonitoring patients or checking vital signs for extended carefacilities, nursing homes, and other health-related facilities

(a12). Blood pressure screening clinics and drugstores will have agreatly proved means of determining patient's blood pressures and othervital signs. Airports d airplanes are able to purchase medicalequipment, but often do not have personnel trained to operate theequipment. The device is simple and quick to operate.

(a13). The patient noted in applications 6 through 9 above can alsomonitor his heart disease and health care at home. The operation of thedevice is straightforward enough to be used by the layman with minimalinstruction, and inexpensive enough for personal home use. The patientcan measure his cardiovascular characteristics daily, or as frequentlyas he and his physician desire. A communication means, such as a modemcan easily be incorporated into the device. This, with appropriatesoftware and support, would allow essentially instantaneouscommunication with a physician's office, clinic, or hospital. Inaddition, a permanent record can be made and stored electronically. Ifdesired, the device could automatically “sign on” to the Internet orother network, and link to the appropriate website or other address. Theability to participate more fully in their own health care will improvethe welfare of individuals.

(a14). The patient of above presents to the emergency room of a hospitalwith chest pain. The ER physician can access, via the Internet or othermeans, the patient's vital sign history, including ECG. This allows thephysician to determine if abnormalities are new or chronic. Changes,such as dysrhythmias, can be identified as to when they first occurred,perhaps to within a time frame of hours or less.

(a15). People without diagnosed cardiovascular disease can use thedevice to allow themselves to participate in their own health care. Thiswill allow virtually immediate diagnosis of any problems, allowing earlyintervention. In addition, a permanent record can be created if desired.

(a16). The device will impact fitness and physical training for everyonefrom lay people to military personnel to professional athletes.

(a17). The device can be employed in the diagnosis and management ofperipheral vascular disease. Measurement of pulse wave velocity in theextremities, and particular differential pulse wave velocities in thelower extremities, can be used to diagnose peripheral vascular disease.Since measurements are real time and continuous, they can also be usedin management. For example, if balloon angioplasty of an artery isperformed, the clinician can tell immediately if flow has improved. Inthe case of angioplasty of coronary arteries, the clinician can followcardiac characteristics on a beat-by-beat basis.

(a18). In addition to peripheral vascular disease, other diseases, suchas abdominal aortic aneurysm, can be diagnosed and managed. Changes inpulse wave velocity and waveform can be followed for years if desired.

(a19). Some of the most important potential uses of the device relate tothe health care of neonates and young children. For these patients, themeasurement of common characteristics such as blood pressure can bedifficult even for highly trained personnel in well-equipped facilities.The simple placement of probes on fingers will alleviate this. Thedevice will also allow noninvasive diagnosis of congenital cardiacdefects and anomalies. Analysis of differential pulse wave velocity andblood pressure will allow rapid, accurate, and specific diagnosis ofmany disorders, including Tetralogy of Fallot and transposition of thegreat vessels. The ability to distinguish both arterial and venoussaturations and pressures will allow diagnosis of patent ductusarteriosus, truncus arteriosus, atrial septal defect, and ventricularseptal defect. Differential arm and leg pulse wave velocities andpressures will confirm diagnosis of coarctation of the aorta. Because ofits continuous measurements, the device can be used for only fordiagnosis but confirmation of adequacy of repair, includingintraoperatively. As the device is inexpensive and easy to operate, itmay become a screening tool for newborns and infants.

(a20). The device can be used in conjunction with intra-aortic balloonpump (IABP) counterpulsation. Beat-by-beat analysis of effectiveness andability to wean from counterpulsation can be made.

(a21). The device can be used in conjunction with placement of cardiacpacemakers, to set proper rate and timing intervals. In addition,efficacy of pacemakers can be checked as frequently as desired, andscheduling of reprogramming or replacement made automatically.

(a22). It is straightforward to incorporate other devices, such as theelectroencephalogram (EEG) or electromyogram (EMG), into probes of theinvention. As a general-purpose monitor, the device will invite theaddition of specialized add-ons.

(a23). Many enhancements are included in the invention. For example,addition of chest (horizontal) leads allows full diagnostic ECGs to beperformed.

(a24). Under some circumstances, such as severe hypotension, the pulsecannot be identified in the periphery. In such cases, many of thedeterminations claimed herein cannot be made. However, the ability ofthe device to identify venous blood can still give importantinformation.

(a25). Forces other than gravity can be used. In a microgravityenvironment such as a space station orbiting the Earth, a device such asthe one described could be constructed to perform all indicateddeterminations using acceleration caused by movement in place ofgravitational acceleration.

(a26). As mentioned in the examples above, an anticipated use is in thefield of home health care, with the possibility of automatic sign-on anddirection to a website. As the user is already participating in his orher health care, the extension of providing access to related health orother information via the Internet® is a natural one.

(a27). A verification means, such as fingerprint scanning, can beincorporated into a personal-use device, to ensure that any medicalinformation gathered belonged to the individual using the device.

(a28). The device will be used in conjunction with the Penaz techniqueor other methods, such as calibration with a cuff or other means, asdesired.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages will become apparent from the followingand more particular description of the preferred embodiments of theinvention, as illustrated in the accompanying drawings, and in whichlike referenced characters generally refer to the same parts or elementsthroughout the views, and in which:

FIG. 1 is a diagram of the central cardiovascular system, showing theasyrmmetry of origins of the vessels off the aortic arch;

FIG. 2 shows a representative probe of the invention with a singleemitter-detector pair;

FIG. 3 shows an alternative embodiment of a probe of the invention witha single emitter-detector pair;

FIG. 4 shows a probe of the invention with two emitter-detector pairsspaced a known distance apart. This can be used to measure the velocityof the pulse wave within the probe itself;

FIG. 5 shows a probe with a single emitter-detector pair and a singleelectrocardiogram (ECG) electrode;

FIG. 6 shows a probe with a single emitter-detector pair and two ECGelectrodes;

FIG. 7 shows a probe with a two emitter-detector pairs and two ECGelectrodes;

FIG. 8 shows a probe of the invention further comprising a positionsensor;

FIG. 9 shows an embodiment of the invention with probes placed onopposite digits of a subject;

FIG. 10 shows an embodiment of the invention with probes placed onopposite temples of a subject;

FIG. 11 shows a circuit schematic of the invention comprising aphotoplethysmogram;

FIG. 12 shows a circuit schematic of the invention comprising aphotoplethysmogram with an ECG amplifier;

FIG. 13 shows a circuit schematic of the invention comprising aphotoplethysmogram with an ECG amplifier and a level signal;

FIG. 14 shows a circuit schematic of the invention comprising aphotoplethysmogram with two independent channels;

FIG. 15 shows a circuit schematic of the invention comprising aphotoplethysmogram with two independent channels and an ECG amplifier;

FIG. 16 shows an embodiment of the invention with probes placed on thedigit and on the arm near the brachial artery;

FIG. 17 shows an embodiment of the invention with probes placed on afinger and on a toe;

FIG. 18 shows an embodiment of the invention with probes placed onopposite fingers and on a toe;

FIGS. 19 and 20 show embodiments of the invention with probes placed onopposite digits of a subject positioned at differential heights relativeto the patient's heart;

FIG. 21 shows an embodiment of the invention with probes placed onopposite fingers positioned at differential heights and on a toe; and

FIGS. 22-25 are graphical representations of an oscilloscope screenshowing recordings using methods of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Functionally the heart is divided into two sides or sections. The rightor pulmonary circulation section that receives blood from the veins ofthe body and pumps it through the lungs and the left or systemiccirculation section that receives the blood from the lungs and pumps itto the body. Th blood is then collected in the veins to be returned tothe right side of the heart. This anatomy is generally shown in FIG. 1.The arterial system begins at the aorta 1, to which the left ventricleof the heart pumps. The first three branches of the aorta are thebrachiocephalic or innominate artery 2, the left (common) carotid artery3, and the left subclavian artery 4. The brachiocephalic artery branchesinto the right subclavian 5 and right (common) 6 carotid arteries. Thesearteries provide the blood supply for the head and upper extremities.The aorta then passes down (caudad) through the body, continuing toprovide arterial branches to organs, terminating as a bifurcationcreating the iliac arteries. The brachiocephalic or innominate artery isthe first branch of the aorta. It in turn branches into the rightsubclavian and right carotid arteries. In contrast, the left subclavianand left carotid arteries originate directly off the aortic arch. Thus,the subclavian and carotid arteries and any of their branches havedifferent paths from their counterparts on the opposite side of thebody.

Because of the different origins from the aorta and different branchingpattern of the arterial tree, it can be appreciated that blood ejectedfrom the left ventricle will not follow symmetrical pathways to oppositearms or opposite sides of the head. Similarly, the pressure pulse waveassociated with left ventricular ejection will follow differentpathways, and can be expected to arrive at different times for pairedorgans or members of the upper body.

Measurements performed by the inventor have shown this delay can rangefrom less than one millisecond to several milliseconds, depending on thesubject and circumstances. In addition, the inventor has found that thisdelay can be altered by several methods disclosed herein. Thispropagation delay, its alterations, and other factors make possibleseveral determinations heretofore not possible by noninvasive means.

Blood pressure is the pressure exerted by the blood within a vessel uponthe wall of the vessel. It is measured in units of force per unit area.Central venous pressure is the pressure within the large veins in thechest and the right atrium, which is the common emptying point for thevenous system. Cardiac output is the amount of blood pumped by theheart, expressed in units of volume per time.

Central venous pressure (CVP) is defined as the distending pressurepresent in the veins in the chest (proximate to the heart), and isconsidered equal to the pressure in the right atrium (which is theemptying point for the venous system). Pressure should be the samethroughout the venous system, but there are valves to ensure that theblood does flow back toward the heart (for example, when standing thevenous blood must flow uphill, and there is no pump as on the arterialside).

As discussed in detail below, the present invention generally includes aradiation emitter having at least one wavelength being applied through apatient's tissue to the patient's blood; a radiation detector whichdetects reception of the at least one wavelength after absorbancethrough the blood, a movement generator for inducing position changes inthe tissue; and a controller for computing the various characteristicsbased on the absorbance of the at least one wavelength of radiation atvarious position levels. In a preferred embodiment, the radiationemitter and detector are inserted in a probe which can be placed aboutthe tissue/blood to be measured. A number of suitable configurations forprobes are shown in FIGS. 2-8.

For example, FIG. 2 shows a representative probe 10 with a singleemitterdetector pair 12. The emitter and detector are placed such thattransmittance through a body member, such as a finger 13, is measured.Generally, any part of the body that can be successfullytransilluminated with the radiant energy used can be utilized. Thus,toes, ears, etc. could also be used. In addition, pulse oximetry can beaccomplished with this and all of the following embodiments. FIG. 3shows a representative probe 14 with a single emitter-detector pair 16placed such that reflectance of a body member, such as a finger, ismeasured. Further, FIG. 4 shows a probe 18 with two emitter-detectorpairs 20 and 22 spaced a known distance apart. This can be used tomeasure the velocity of the pulse wave within the probe itself.

In certain embodiments of the invention, the probe comprises one or moreelectrocardiogram (ECG) electrodes in conjunction with theemitter-detector pairs. For example, FIG. 5 shows a probe 24 with asingle emitter-detector pair 26 and a single electrocardiogram (ECG)electrode 28. Similarly, FIG. 6 shows a probe 30 with a singleemitter-detector pair 32 and two ECG electrodes 34 and 36 and FIG. 7shows a probe 38 with a two emitter-detector pairs 40 and 42 and two ECGelectrodes 44 and 46. Such probes, if placed on opposite extremities ofa patient, can be used to measure central and peripheral pulse wavevelocity as well as ECG. Other configurations, such as doubleemitter-detector pairs and single ECG electrode, can be envisioned.

In yet other embodiments of the invention, the probe further comprises aposition sensing or measuring device together with the emitter-detectorpairs and/or ECG electrodes. FIG. 8 shows a probe 48 similar to thatshown in FIG. 7 with the addition of a position sensor 50. This positionsensor could be used in conjunction with a position sensor placed atheart level in order to determine the hydrostatic pressure differencebetween the two position sensors.

As discussed in detail herein, the invention employs hydrostaticpressure to enable precise self-callibration of the devices in acompletely non-invasive manner. Hydrostatic pressure affects allliquids. Gravity or other acceleration will affect both the arterial andvenous sides of the circulation. It affects all aspects of the bloodpressure equally—mean, systolic, diastolic. For example, an increase inheight which causes a change of 10 torr will change every pressuremeasurement during the cardiac cycle by this amount.

For example, if the “true” blood pressure (taken level with the heart)is 120/80, when the arm is raised an amount needed to decrease themeasured pressure by 10 torr, the measured pressure in the arm will be110/70. The pulse pressure will be the same, but the transmural pressurewill be 10 torr lower at all times. In addition, the vessel will besmaller at all points.

The heart is taken to be the center of the circulatory system, and allvalues are in reference to it. This is not necessary for the practice ofthe invention, but serves as reference points for values in the currentmedical literature.

The electromagnetic radiation in this description will refer to light inthe visible and infrared range although, as noted in the attachedclaims, it is conceivable that other forms could be used.

Similarly, while the present invention primarily describes the use oftransillumination, it will be appreciated that reflectancespectrophotometry may alternatively be employed.

Operating Principles

It is well known that Incident radiation passing through a body part isattenuated (absorbed) in the tissue. The theoretical basis forspectrophotometic techniques is Beer's law (the Beer-Lambert-Bouguerlaw) which expresses the incident intensity in terms of transmittedintensity and extinction coefficients of the tissue compartments throughwhich the radiation has passed. The equation can be written as:

ln(l/lo)=E*C*L  Eq.1

Where:

Io=the incident intensity of the source radiation;

I=the transmitted intensity of the source through the sample;

E=the extinction coefficient of the component of interest;

C=the concentration of the component in the tissue itself;

L=the optical path length (distance) through the absorber; and

E*C*L=absorbance.

Beer's law and the practice of spectrophotometry and oximetry have beenexhaustively reviewed in the literature. Generally, pulse oximetry ineffect filters out signals other that pulsating (AC). In the body, itcan be assumed that the pulsatile component of the signal is arterialblood, while all other tissue absorbers should be non-pulsatile (DC).

An additional feature of this invention, not found in any previousdisclosure, is the use of hydrostatic pressure changes to vary theamount of venous blood within a body member such as a finger. Thus,hydrostatic changes can be used in a similar manner to the pulse toperform measurements on both arterial and venous blood. If a finger iscontained within a probe, raising the probe will lower the hydrostaticpressure of all vessels in the finger, both arterial and venous. Botharteries and veins (and arterioles and venules) will be smaller due tolower pressure distending their walls. Most change will occur on thevenous side of the circulation due to lower pressure. Total absorbanceof the finger will decrease. As the arterial oxygen saturation can bemeasured by pulse oximetry, the venous oxygen saturation can becalculated in a similar manner.

A light signal of a known intensity and wavelength is produced by meansof light-emitting diodes (LEDs) as in currently used oximeters or, as inone possible embodiment, a broad-band light source whereby wavelengthsare isolated by a rotating filter or diffusion grating. In the lattercase, the emitted light is distilled through a filter which allows aknown wavelength and intensity of light to penetrate. Use of tunablelasers or other equipment is also possible. If the light source isproximate to the point of use, no further mode of transmission will beneeded. If it is not, the light will be transported to the desired pointby means such as a fiber optic cable, preserving the wavelength andintensity.

Several means of motion induction are possible. Various means ofposition measurement are also possible. For example, a liquid filledtube with an end open to the atmosphere can be employed. Other positionsensors are known to those having skill in the art, and includeelectromagnetic, spectroscopic, and chemical means. A broad-band photodetector (in the case of visible or infrared light) or other means willbe utilized to measure the quantity of transmitted light.

To generate a single data point, the movement induction means is used tobring the finger (or other space of interest) to a known positionrelative to the heart. Light of known wavelength and intensity isemitted (and transmitted if necessary) on the surface of interest.Detection of the light signal at a distinct point (normally opposingsurface) is made and the relative absorbance and extinction of thesignal is calculated. Signal processing is used to determine thepulsatile portion of the signal. The arrival time of the pulse isrecorded, as is the amplitude and waveform. This measurement may berepeated one or more times to ensure the accuracy of the measurement;this can be done within a very short time frame (less than amillisecond).

To generate multiple data points, the process outlined in the previousstep will be repeated at the next chosen wavelength, while still at thesame predetermined position. The range and number of wavelengths can beselected, and changed for different applications.

Once the desired number of wavelengths has been examined, the movementinduction means would bring the finger or other volume to apredetermined second position, and the data collection of steps would berepeated. At the completion of measurements and determinations for thissecond position, the movement induction means will bring the space to athird predetermined position, and the measurements and determinationsrepeated. This process would be continued until the desired range ofpositions has been scrutinized.

In order to make computations of pulse propagation delay, identicalmeasurements would be made simultaneously with a probe on the samemember on the opposite side of the body. For example, if one probe wereplaced on the index finger of the right hand, the other probe would beplaced on the index finger of the left hand.

Because the arterial path to the arm is essentially identical after thesecond part of the subclavian artery, any differences in pulse wavevelocity and pulse wave propagation time must occur prior to this point;that is, very close to the root of the aorta. In any case, pulse wavevelocity increases rapidly as the pulse wave propagates down the aortaand into the periphery (Fung). Thus, any timing differences in theperiphery will be greatly reduced by the high wave velocity, leavingcentral effects as the most prominent.

The apparatus of the invention can be operated intermittently orcontinuously. In the intermittent mode, a single set of calculations canbe used for analysis to produce the determinations claimed. However, thedevice can also be easily operated in continuous mode, with the processoutlined above repeated as often as wished (constantly if desired). Inaddition, a rapid (“stat”) mode can be offered with the minimum numberof measurements made that will provide an accurate estimation of correctvalues. Such a rapid mode would be useful in emergency situations.

While this methodology should give precise values, further adjustmentmay be desired to compensate for any discrepancies between theoreticaland in vivo measurements. Contemporary oximeters in fact use acalibration curve when determining oxygen saturation, with the curvebeing generated with data from normal volunteers.

Calculations and Analysis

The following algorithms are further examples of the use of the presentinvention. Some variables have degrees of co-dependence. In these cases,values are calculated by iterative computational techniques.

Generally, measurement of pulse wave amplitude and timing is made usingprobes such as that shown in FIG. 2, using methods similar to standardoximetry described in the prior art. As shown in FIG. 9, a first probe52 is placed on a finger and set at a known position relative to theheart. Another, simultaneous measurement of pulse wave amplitude andtiming is made by a second probe 54 placed on a finger on the handopposite that of the first probe. The pulse delay occurring between thetwo measurements is made. Alternatively, as shown in FIG. 10, probes 52and 54 can be placed on opposite temples of the patient to measure pulsewave values and delay. The probes can also be placed on the patient'sears.

From this information alone, an estimate of pulse wave velocity at theaortic root could be made, by utilizing a table of normal values for thedistance of the central anatomical difference.

If a measurement of blood pressure is then made, one can perform thefollowing calculation:

p=c*u*ρ  Eq.2

Where:

c=pulse wave velocity;

u=flow wave velocity; and

ρ=the density of the blood (approximately 1.055 grams/cm³).

According to the invention, p and c have been measured, ρ is known. Thisallows one to solve for u, which is the flow wave velocity at the aorticroot. This by itself is a measure of cardiac output. If one makes anestimate of aortic root diameter, one can then compute cardiac strokevolume.

Techniques described by O'Rourke and others describe reconstructiontechniques that can be used to convert or “transform” peripheral bloodpressures and waveforms to the corresponding pressure and waveform atthe aortic root. Ideally, the blood pressure at the aortic root shouldbe used as the pressure term in Fung's equation.

One can improve on the above determination in several ways. The firstway is by additionally measuring the peripheral pulse wave velocity. Todo this, measurement of pulse wave amplitude and timing is made by afirst probe such as that shown in FIG. 5. The probe is at a set knownposition relative to the heart. Another, simultaneous measurement ofpulse wave amplitude and timing is made by a second probe placed on afinger on the hand opposite that of the first probe. The pulse delayoccurring between the two measurements is made. The respectiveperipheral pulse wave velocities are also computed. If the peripheralpulse wave velocities are different, it can be assumed that this isbecause of the different central anatomies from which the respectivepulses traveled. This information alone may be enough to compute centralpulse wave velocity from a table of normals. However, when combined withthe pulse wave delay information, this data enables one to construct afunction of pulse wave speed from the periphery back to the aortic root,thus giving another measure of central pulse wave velocity.

Another method of the invention is to vary the position of the probesrelative to the heart. If the first probe is at heart level and thesecond probe is raised above (with respect to the earth) heart level,the hydrostatic pressure of the blood vessels within the second probewill be lower than those within the first probe. In turn, in accordancewith Fung's equation stated above, this means that the pulse wavevelocity of the arterial vessels within the second probe will be lowerthan that in the arterial vessels within the first probe. This willchange both the measured pulse delay between the two probes, and themeasured peripheral pulse wave velocities. This creates additionalmeasurements by which to compute central pulse wave velocity.

According to the invention, changes in hydrostatic pressure arecontrolled by the following equation:

p=ρ*g*h  Eq.3

Where:

ρ=blood density;

g=gravitational acceleration; and

h=height above a reference point (with respect to the earth).

The difference in hydrostatic pressure between the vessels in two probesis thus governed completely by their difference in heights relative tothe heart (referenced to the surface of the earth). Therefore, a knownchange in position produces a known change in hydrostatic pressure.

According to the invention, the above measurements can be employed toderive a number of physiological properties. Preferably, the probes ofthe invention are connected to a controller to aid the data collectionand analysis used to make the desired determination. The controllerincludes a computing device or standard personal computer (PC) with amonitor. Included within the controller are algorithms for thecalculation of variables not measured directly.

For example, FIG. 11 shows a circuit schematic for a one or twowavelength photo-plethysmograph. Emitters 56 and 58 and detector 60 arepositioned adjacent the tissue being measured, such as a finger 61.Emitters 56 and 58 are driven by drive circuitry 62, which is in turngoverned by control signal circuitry 64. Detector 60 is connected toamplifier 66. The signal from amplifier 66 is sent to demodulator 68,which is also synched to control signal circuitry 62. The signal fromthe demodulator 68 is sent to analog-digital converter 70. The desiredcomputations are performed on the output from the converter 70 by signalprocessor 72 and the results sent to display 74. Emitters 56 and 58operate specific wavelengths, such as 805 nm, and may comprise lightemitting diodes (LEDs) or laser diodes. Detector 60 preferably comprisesa silicon photodiode. Such emitter-detector pairs are shown in FIGS. 2and 3.

FIG. 12 shows a schematic of an alternate embodiment of suitablecircuitry. As with FIG. 10, emitters 76 and 78 are connected via LEDdrive circuitry 79 and control signal circuitry 80 to demodulator 82.Signal from detector 84 is amplified at circuit block 86 and sent todemodulator 82. Output from demodulator 82 is sent to A/D converter 88.In addition, ECG leads 90 are connected to differential amplifier 92 andthe signal is sent to converter 88. Output from converter 88 isprocessed at block 94 and the results sent to display 96. A probe suchas those shown in FIGS. 5 and 6 may be used with the circuitry. The ECGleads are preferably silver/silver chloride or stainless steel.

Yet another embodiment of the invention is shown in FIG. 13. Emitters 98and 100 are connected via LED drive circuitry 101 and control signalcircuitry 102 to demodulator 104. Signal from detector 106 is amplifiedat circuit block 108 and sent to demodulator 104. Output fromdemodulator 104 is sent to A/D converter 109. ECG leads 110 areconnected to differential amplifier 112 and the signal is sent toconverter 109. Digit level sensor 114 and heart level sensor 116 areconnected to amplifier 118 and the signal is sent to converter 109.Output from converter 109 is processed at block 120 and the results sentto display 122.

FIG. 14 shows a circuit schematic suitable for use with a probe havingtwo physically independent channels, such as the one shown in FIG. 4. Afirst emitterdetector pair comprising emitters 124 and 126 and detector128 are positioned adjacent the tissue being measured, such as a finger.A second pair comprising emitters 132 and 134 and detector 136 arepositioned a selected distance from the first pair. Emitters 124, 126,132 and 134 are driven by drive circuitry 138, which is in turn governedby control signal circuitry 140. Signal from detector 128 is amplifiedby block 142 and sent to demodulator 144. Independently, signal fromdetector 136 is amplified and demodulated at blocks 146 and 148,respectively. Output from demodulators 144 and 148 is sent toanalog-digital converter 150. The desired computations are performed onthe output from the converter 150 by signal processor 152 and theresults sent to display 154.

An alternative embodiment configured for use with a probe having twophysically independent channels and an ECG lead, such as the one shownin FIG. 7, is schematically shown in FIG. 15. A first emitter-detectorpair comprising emitters 156 and 158 and detector 160 are positionedadjacent the tissue being measured, such as a finger. A second paircomprising emitters 164 and 166 and detector 168 are positioned aselected distance-from the first pair. Emitters 156, 158, 164 and 166are driven by drive circuitry 170 which is in turn governed by controlsignal circuitry 172. Signal from detector 160 is amplified by block 174and sent to demodulator 176. Independently, signal from detector 168 isamplified and demodulated at blocks 178 and 180, respectively. Outputfrom demodulators 176 and 180 is sent to analog-digital converter 182.ECG leads 184 are connected to differential amplifier 186 and the signalis also sent to converter 182 The desired computations are performed onthe output from the converter 182 by signal processor 188 and theresults sent to display 190.

As one of ordinary skill in the art will appreciate, the placement ofthe various probes discussed above will effect the types of measurementsthat can be taken. As discussed above, FIGS. 9 and 10 show probes placedon opposite extremities to enable measurement of pulse wave delay. FIG.16 shows an embodiment of the invention with probe 52, such as in FIG.1, placed on the digit, and a probe 54, such as in FIG. 2, placed on thearm near the brachial artery. This could measure the pulse wave velocityin the arm (as well as pulse oximetry). A similar embodiment couldmeasure pulse wave velocity in the leg. FIG. 17 shows probes 52 and 54placed on a finger and on a toe to measure the pulse wave delay. FIG. 18shows probes 52 and 54 placed on opposite digits and probe 55 placed ona toe. This allows measurement of the differential pulse wave delaybetween the fingers and toe, and allows calibration of the toe probe tobe used in place of a finger probe (if only one finger probe could beused, such as in hand surgery). The use of appropriate probes alsoallows a diagnostic-quality ECG. FIGS. 19 and 20 show probes 52 and 54placed on opposite digits. One arm of the subject is placed at the levelof the heart, while one arm is moved to different positions, both aboveand below the level of the heart. By generating different hydrostaticpressures in the vessels, the pulse velocity and hence pulse wave delaychanges. In addition, the amplitude of the pulse wave, and amplitude ofvenous absorbance changes. This allows the additional computations ofarterial blood pressure and venous pressure. FIG. 21 shows probes 52 and54 placed on opposite digits and probe 55 placed on a toe. Thedifferential hydrostatic pressures in the vessels allow measurements ofpulse wave velocity and pulse wave delay, as well as arterial bloodpressure and venous pressure. Use of probes with suitable ECG leads willalso allow the invention to perform a diagnostic-quality ECG. Inaddition, heart rate and respiratory rate can be calculated, and cardiacoutput and several other cardiovascular characteristics computed.

As discussed above, the controllers of the invention preferably outputthe results of the measurements and computations to a display. FIG. 22shows an oscilloscope screen. The two tracings are from pulse oximeterprobes, such as those shown in FIG. 1, placed on the index fingers ofboth hands. The pulse wave delay is visible as the slight phasedifference between the two tracings. As the probes are at the samelevel, the pulse amplitudes are essentially identical. FIG. 23 shows theoscilloscope screen after the hand with the probe displayed as the toptracing has been placed at a level higher than the heart and the handwith the probe displayed as the bottom tracing has been placed at alevel lower than the heart. The induction of a pressure differentialbetween the two probes effects a change in the pulse delay. The changein pressure also correspondingly alters the pulse amplitudes. FIG. 24shows the oscilloscope screen after the hand with the probe displayed asthe top tracing has been placed at a level lower than the heart and thehand with the probe displayed as the bottom tracing has been placed at alevel higher than the heart. Here, the pulse delay has substantiallyreversed as have the pulse amplitudes. FIG. 25 shows an oscilloscopescreen displaying an electrocardiogram in conjunction with a pulsewaveform.

The algorithms outlined below serve as examples, but modifications arepossible to arrive at the indicated results, and are meant to beincluded within the spirit of this application. Various additionalcomponents of the device will be discussed in more detail below withreference to the following examplary determinations.

(d1). Determination of Arterial Blood Pressure

A probe such as that shown in FIG. 1 is placed on an extremity, and thatextremity is moved in relation to the heart. As mentioned above, thehydrostatic pressure within the arteries and arterioles changes as afunction of height with respect to the heart. Because of this, both thepulse wave velocity and pulse wave amplitude change as a function ofprobe height. These two parameters can be mapped against known distanceabove or below the heart. In this way, function curves of pressure vs.pulse wave amplitude and pressure vs. pulse wave velocity can be drawn.For example, a full excursion of the arm in a standing adult produceshydrostatic changes of greater than 50 cm of water in both directions.Using an arm and a leg, a gradient of well over 200 cm of water can begenerated. This is a significant portion of the normal blood pressurerange, and certainly enough to produce the function curves mentionedabove.

There is a huge amount of medical literature describing arterialbehavior, so the curves can be extrapolated if necessary. These curvesserve as calibration.

It can thus be determined if “recalibration” is necessary—if eitherpulse amplitude or pulse wave velocity changes, and the other parameterdoes not change correspondingly. In other words, a shift on one curveshould matched by a corresponding shift on the other curve. If thisshift does not occur as predicted, recalibration is required. Of course,the process of recalibration is the simple procedure outlined above.

In a preferred embodiment, a first probe having a position sensor isplaced level with the patient's heart. A second probe, such as one shownin FIG. 8, having a position sensor and a pulse detector is placed onthe patient's finger. The patient's arm is held out level with the heartso there is zero displacement between probes. Pulse amplitude isrecorded from probe. The patient's arm is slowly raised, while pulseamplitude and relative displacement of probe are recorded. Thehydrostatic pressure difference between probes is also computed. Bycomparing the recorded pulse amplitude to the hydrostatic pressuredifference, a mathematical function relating pressure to pulse amplitudecan be derived. Preferably, circuitry similar to that shown in FIG. 13is used to aid the process. This process is repeated while lowering thearm back to heart level, then lowering the arm to below heart level and,finally, raising the arm back to heart level. Similar steps can beapplied to measure pulse delay, pulse velocity and pulse contour.

(d2). Determination of Cardiac Output

Cardiac output can be determined by measuring delays in pulse arrivaltimes in coupled organs or members on opposite sides of the body. In apreferred embodiment of the invention, probes such as those shown inFIG. 1, having sensors for detecting a patient's pulse are placed onopposite fingers of the patient. The patient positions both armsstraight out from the side. The blood pressure of the patient can bedetermined either through conventional means or by the methods of theinvention. The pulse delay between the two probes can be measuredutilizing circuitry such as that shown in FIGS. 14 or 15, for example.The dicrotic notch of the pulse may be determined by standard methods,and used to calculate the ejection time based on the timing. The size ofthe aortic root can be estimated by standard means and the consequentlythe pulse distance differential at the aortic root. This allows thecalculation of the pulse velocity c at the aortic route by the followingequation:

c=(pulse distance)/(pulse delay)  Eq.4.

The value of c can then be used to solve for flow wave velocity based onthe following equation:

p=c*u*ρ  Eq.5

where

c=pulse wave velocity;

u=flow wave velocity; and

ρ=density of the blood (approximately 1.055 grams/cm³).

According to the invention, cardiac stroke volume can be determined bymultiplying the aortic root area by the flow wave velocity and by thecardiac ejection time. Cardiac minute output can be calculated bymultiplying the cardiac stroke volume by the pulse rate. These steps maybe augmented by raising and lowering the patient's arms with respect toeach other to vary the pressure and the pulse wave velocity.

Alternatively, cardiac output can be determined by placing probes suchas those shown in FIG. 5 on a patient's finger and toe. The probesmeasure oxygen saturation at each pulse. The oxygen saturation for eachpulse at the first probe is compared to the oxygen saturation of thatpulse and subsequent pulses at the second probe. With continuousmonitoring, this allows the determination matching oxygen saturation,within given tolerance limits, of the pulses from the probes. Thepatient's blood volume and the physical separation of the probes can bedetermined by standard methods. This allows the computation of caridacstroke volume by dividing the blood volume displaced by the number ofpulses. Then, the cardiac minute output can be calculated by multiplyingthe cardiac stroke volume by pulse rate. Circuitry such as that shown inFIGS. 11 or 12 is suitable for use with this embodiment.

(d3). Determination of Venous Saturation and Pressure

Determination of arterial oxygen saturation can be determined by pulseoximetry and techniques well delineated in both the patent and medicalliterature. Hydrostatic changes as described in this application allowthe determination of venous saturation and pressure as well.

Place a probe such as that shown in FIG. 1 on a finger. Makemeasurements of both total absorbance and pulsatile absorbance. Raisethe probe a known distance. Again measure both total absorbance andpulsatile absorbance. Both will be decreased. This is because the pulseamplitude is less because the arterial blood pressure within the probeis less (due to decrease in hydrostatic pressure). However, the totalabsorbance will also decrease, as the distending pressure in the venoussystem is less, and hence the veins and venules are smaller. All changesin absorbance can be assumed to be due to changes in blood volume.Saturation is calculated using the ratios of absorbance of distinctwavelengths.

In one embodiment, the central venous pressure (CVP) can be estimated. Aprobe containing a position sensor is place level with a patient'sheart. A second probe, such as the one shown in FIG. 8, also comprisinga position sensor is placed on the patient's finger. The patientpositions the arm so that the second probe is initially lower than thefirst probe. The total absorbance measured at the second probe iscontinuously monitored. The patient's arm is slowly raised, and the rateof change of absorbance of the second probe is computed with respect tothe relative displacement to the first probe. When the rate of changechanges by a predetermined amount representing an abrupt decrease, thearm position corresponding to the point of central venous drainage hasbeen reached. The CVP can then be calculated by computing thehydrostatic pressure difference between the first probe and the secondprobe at that arm position. The circuitry shown in FIG. 13 is suitablefor use with this embodiment.

(d4). Determination of Heart Rate

According to the invention, heart rate can be determined by counting thepulsatile arterial signal for a known length of time, or by the ECGimpulse.

(d5). Determination of Respiratory Rate

The impedance changes of the chest due to filling and emptying can bemeasured from the electrocardiogram tracing. During normal breathing,negative pressure is created within the chest by lowering of thediaphragm and expansion of the rib cage. This negative pressure causesblood to empty more rapidly from the peripheral into the central veins.This is also the case when respiration is assisted by anegative-pressure device such as the “iron lung”.

During modem mechanically-assisted ventilation (with “ventilators”),positive pressure is created within the chest by forcing air into thelungs. For both positive- and negative-pressure ventilation, expirationis passive. This respiratory variation by itself can be used as anestimate of cardiac filling, giving left heart pressures. Thisdetermination can be assisted by the use of the hydrostatic techniquesdescribed above.

(d6). Diagnosis of Congenital Heart Disease and Anatomic Anomalies

Diagnosis of many disorders with anatomic anomalies can be made by thedetection of unexpected propagation times, and abnormal propagationdelays between right- and left-sided organs.

The ability to measure both arterial and venous saturation, as well asarterial and venous pressures, can aid further in investigations.

(d7). Diagnosis of Dysrhthmias

By measuring blood pressure and the electrocardiogram simultaneously,the diagnosis of dysrhythmias can be aided greatly. Both arterial andvenous pressure are recorded with the ECG, allowing differentiation ofatrial vs. ventricular arrhythmias.

(d8). Determination of Additional Cardiovascular Characteristics

By measuring blood pressure and the electrocardiogram simultaneously,many additional characteristics, such as systolic and diastolic pressuretime indices, can be determined.

An enormous amount of information can be gleaned from the use of probeson opposite sides of the body combined with hydrostatic perturbations.It is important to realize that the time of arrival of a pulse to pairedmembers is different, but the velocity of the pulse is also different.Examination of pulse propagation time, pulse propagation phase or delay,pulse velocity, and pulse amplitude yields four parameters that maychange in different ways for each perturbation. Particularly, raisingand lowering an arm by the same amount may give different changes.Raising and lowering the other arm by the same amount may give stilldifferent changes. Further, raising an arm by a given amount, thenraising again by the same amount, may give different changes. Raisingthe other arm by the given amount, then raising again by the sameamount, may give still different changes. Similar effects can beobtained by lowering the extremity.

Without departing from the spirit and scope of this invention, one ofordinary skill can make various changes and modifications to theinvention to adapt it to various usages and conditions. As such, thesechanges and modifications are properly, equitably, and intended to be,within the full range of equivalence of the following claims.

What is claimed is:
 1. A device for the noninvasive monitoring of aphysiologic characteristic of a patient's blood, comprising: a tissueprobe having a first radiation emitter with a first wavelength and afirst radiation detector configured to receive the first wavelengthafter absorbance through the patient's blood; a position sensor fordetermining the relative height compared to a level corresponding to thepatient's heart; and a controller for computing the physiologiccharacteristic of the patient's blood based on pulse wave amplitudecalculated from the absorbance of the first wavelength of radiation andthe relative height of the probe.
 2. The device of claim 1, wherein thefirst wavelength is selected from the group consisting of visible light,infrared light, and ultraviolet light.
 3. The device of claim 1, whereinthe probe is configured to monitor tissue selected from the groupconsisting of hands, fingers, feet, toes, ears, earlobes, nares, lips,and tongue.
 4. The device of claim 1, wherein the tissue probe furthercomprises a second radiation emitter with a second wavelength and asecond radiation detector configured to receive the second wavelengthafter absorbance through the patient's blood.
 5. The device of claim 4,wherein the second wavelength is different than the first wavelength. 6.The device of claim 1, wherein the device further comprises a secondprobe having a second radiation emitter with a second wavelength and asecond radiation detector configured to receive the second wavelengthafter absorbance through the patient's blood and wherein the controllerfurther computes the physiological characteristic by comparing theabsorbance detected by the first detector and the second detector. 7.The device of claim 1, wherein the probe further comprises at least oneelectrocardiogram lead.
 8. A device for the noninvasive monitoring of aphysiologic characteristic of a patient's blood, comprising: a tissueprobe having a first radiation emitter with a first wavelength and afirst radiation detector configured to receive the first wavelengthafter absorbance through the patient's blood secured to a desiredportion of the patient's tissue; a movement generator for inducing aposition change of the probe with respect to a level corresponding tothe patient's heart; and a controller for computing the physiologiccharacteristic of the patient's blood based on the absorbance of thefirst wavelength of radiation and the relative position of the probe. 9.The device of claim 8, wherein the movement generator induces a knownposition change of the probe.
 10. The device of claim 8, wherein thetissue probe further comprises a second radiation emitter with a secondwavelength and a second radiation detector configured to receive thesecond wavelength after absorbance through the patient's blood.
 11. Thedevice of claim 8, wherein the device further comprises a second probehaving a second radiation emitter with a second wavelength and a secondradiation detector configured to receive the second wavelength afterabsorbance through the patient's blood and wherein the controllerfurther computes the physiological characteristic by comparing theabsorbance detected by the first detector and the second detector. 12.The device of claim 8, wherein the probe further comprises at least oneelectrocardiogram lead.
 13. A method for noninvasively determining aphysiological characteristic of a patient's blood comprising the stepsof: providing a tissue probe having a first radiation emitter with afirst wavelength and a first radiation detector configured to receivethe first wavelength after absorbance through the patient's blood;measuring absorbance of the patient's blood by emitting a firstradiation through the patient's blood and detecting the radiation afterpassage through the patient's blood with the probe at a first positionrelative to a level corresponding to the patient's heart; computing ablood parameter at the first position based on the absorbance; movingthe probe relative to a level corresponding to the patient's heart to asecond position; measuring absorbance at the second position; computingthe blood parameter based on the absorbance at the second position; anddetermining the physiological characteristic by comparing the absorbanceat the first and second position.
 14. The method of claim 13, furthercomprising the steps of: computing the hydrostatic pressure differencebetween the first and second position; and performing self-calibrationbased upon the hydrostatic pressure difference.
 15. The method of claim13, further comprising the steps of: computing the hydrostatic pressuredifference between the first and second position; comparing the bloodparameter to the hydrostatic pressure difference; and deriving amathematical function relating hydrostatic pressure to the bloodparameter.
 16. The method of claim 15, wherein the physiologicalcharacteristic comprises arterial blood pressure.
 17. The method ofclaim 15, wherein the blood parameter is selected from the groupconsisting of pulse amplitude, pulse delay, pulse velocity and pulsecontour.
 18. A method for noninvasively determining a physiologicalcharacteristic of a patient's blood comprising the steps of: providing afirst tissue probe having a first radiation emitter with a firstwavelength and a first radiation detector configured to receive thefirst wavelength after absorbance through the patient's blood; providinga second tissue probe having a second radiation emitter with a secondwavelength and a second radiation detector configured to receive thesecond wavelength after absorbance through the patient's blood;positioning the first and second probes at coupled tissue locations onthe patient; measuring absorbance of the patient's blood at the coupledtissue locations by emitting radiation through the patient's blood anddetecting the radiation after passage through the patient's blood;determining a blood parameter by comparing absorbance at the coupledtissue locations; and computing the physiological characteristic of thepatient's blood.
 19. The method of claim 18, further comprising thesteps of: measuring blood pressure; determining the pulse delay bycomparing absorbance at the coupled tissue locations; estimating thepulse distance differential; and computing pulse velocity from pulsedistance and pulse delay.
 20. A method for noninvasively determining aphysiological characteristic of a patient's blood comprising the stepsof: providing a first tissue probe having a first radiation emitter witha first wavelength and a first radiation detector configured to receivethe first wavelength after absorbance through the patient's blood;providing a second tissue probe having a second radiation emitter with asecond wavelength and a second radiation detector configured to receivethe second wavelength after absorbance through the patient's blood;positioning the first and second probes at opposing locations on thepatient; measuring absorbance of the patient's blood at the opposinglocations by emitting radiation through the patient's blood anddetecting the radiation after passage through the patient's blood;determining a blood parameter by comparing absorbance at the locations;and computing the physiological characteristic of the patient's blood.21. The method,of claim 20, wherein the step of positioning the firstand second probes comprises positioning the probes at opposing locationson the patient, further comprising the steps of: measuring bloodpressure; determining the pulse delay by comparing absorbance at theopposing locations; estimating the pulse distance differential; andcomputing pulse velocity from pulse distance and pulse delay.
 22. Amethod for noninvasively determining a physiological characteristic of apatient's blood comprising the steps of: providing a first tissue probehaving a first radiation emitter with a first wavelength and a firstradiation detector configured to receive the first wavelength afterabsorbance through the patient's blood; providing a second tissue probehaving a second radiation emitter with a second wavelength and a secondradiation detector configured to receive the second wavelength afterabsorbance through the patient's blood; positioning the first and secondprobes at locations on the patient; measuring absorbance of thepatient's blood at the first and second probe locations by emittingradiation through the patient's blood and detecting the radiation afterpassage through the patient's blood; determining a blood parameter bycomparing absorbance at the locations; moving the first probe relativeto a level corresponding to the patient's heart to a second position;measuring absorbance at the second position; computing the bloodparameter based on the absorbance at the second position; anddetermining the physiological characteristic by comparing the absorbanceat the first and second position and the absorbance at the first andsecond probes.
 23. The method of claim 22, wherein the step ofdetermining the physiological characteristic comprises determiningarterial blood pressure by computing pulse velocity.